docs: def add_manifold_attribute(cls, name):
          """ Register a manifold engine attribute.
  
              @return: field definition object, or None if "name" isn't a manifold attribute.
          """
          if name.startswith("custom_"):
              try:
                  return FieldDefinition.FIELDS[name]
              except KeyError:
                  field = OnDemandField(fmt.to_unicode, name, "custom attribute %r" % name.split('_', 1)[1],
                      matcher=matching.PatternFilter)
                  setattr(cls, name, field) # add field to all proxy objects
  
                  return field
          elif name.startswith("kind_") and name[5:].isdigit():
              try:
                  return FieldDefinition.FIELDS[name]
              except KeyError:
                  limit = int(name[5:].lstrip('0') or '0', 10)
                  if limit > 100:
                      raise error.UserError("kind_N: N > 100 in %r" % name)
                  field = OnDemandField(set, name,
                      "kinds of files that make up more than %d%% of this item's size" % limit,
                      matcher=matching.TaggedAsFilter, formatter=_fmt_tags,
                      engine_name="kind_%d" % limit)
                  setattr(cls, name, field)
  
                  return field